The present invention relates to the field of digital radiocommunications between base stations and mobile terminals, using time-division multiplexing of logical channels on a carrier frequency.
In these systems, the base stations transmit to the mobile terminals frequency- and time-synchronization information, as well as system information allowing the mobile terminals to initiate communication with them if necessary.
Patent Application EP-A-0 896 443 describes a system of radiocommunication with mobile terminals exhibiting the feature of offering time-division multiplexing services with various degrees of protection related to the possible use of a coded modulation on the carrier. For a given throughput, offered for the execution of the service, the number of timeslots allotted to the service is related to the coding or to the absence of coding of the modulation, and/or to the applied coding rate. In a particular embodiment of this system, one and the same service can be offered in a first mode on a half-channel with non-coded modulation, or in a second mode on a full channel with a coded modulation having a code rate of ½.
Half-channels exhibit a structure similar to that of a logical channel with time-division multiplexing of order two: one timeslot out of two is occupied by the service in the first mode. This mode allows in particular time-duplex operation, but this makes it necessary for the terminal to be provided with two frequency synthesizers operating alternately, since a single synthesizer cannot generally switch from the transmission frequency to the reception frequency within the short time span separating two successive slots. This duplication of the frequency synthesizer is not necessary in the second mode. A simplified version of the terminal can be designed, generally with a single synthesizer, not offering the same frequency switching speed characteristics, and therefore not allowing the above time-duplex operation.
Nevertheless, it is important that, in the timeslots devoted to the monitoring of the base stations of neighboring cells, these simplified terminals should be capable of decoding the same information as the terminals having a maximum switching speed.
An object of the present invention is to present a structure of the timeslots devoted to the synchronization and to the broadcasting of the main system data, which allows correct processing thereof, including by terminals having a limited frequency switching speed.